Field of the Invention
The present invention relates to an optical scanning apparatus, which is suitable for an image forming apparatus such as a laser beam printer, a digital copying machine, or a multi-function printer.
Description of the Related Art
In order to record image information with high precision, an optical scanning apparatus mounted in an image forming apparatus such as a laser beam printer is required to have field curvature satisfactorily corrected over the entire area of a scanned surface and have a uniform spot diameter, as well as to have distortion (fθ characteristics) in which an angle of incident rays and an image height have a proportional relationship.
In a sub-scanning section of a scanning optical system of the optical scanning apparatus, there is a need to cause a diverging light flux from a deflection unit to form an image on the scanned surface at a predetermined magnification, with the result that a refractive power is stronger than in a main scanning section, and aberrations generated in the scanning optical system is thus difficult to correct.
Further, along with reductions in size and cost of the image forming apparatus, the optical scanning apparatus is also required to be reduced in cost.
In each of Japanese Patent Application Laid-Open No. H08-76011, Japanese Patent Application Laid-Open No. 2003-337295, and Japanese Patent Application Laid-Open No. 2007-45094, there is proposed means for realizing both the demands of satisfactory correction of the aberrations and the like and the reduction in cost in an optical scanning apparatus.
In Japanese Patent Application Laid-Open No. H08-76011, there is proposed an optical scanning apparatus in which a scanning optical system is formed of one toric lens to satisfactorily correct field curvature, the distortion, and the like, and to reduce influences of jitter caused by an error in manufacturing a deflection unit and a change in spot diameter in a sub-scanning direction depending on an image height.
In Japanese Patent Application Laid-Open No. 2003-337295, there is proposed an optical scanning apparatus suitable for high-definition printing, in which, in order to reduce influences of changes in an environment such as temperature and the like on performance, a scanning optical system is formed of one lens, and an incident optical system has a diffraction optical surface.
In Japanese Patent Application Laid-Open No. 2007-45094, there is proposed a scanning optical system capable of improving yield even when the number of parts is reduced.
In order to realize the reduction in cost in the optical scanning apparatus, it is effective to use a toric lens manufactured by plastic injection molding in the scanning optical system, and hence to increase the number of lenses obtained in one injection molding cycle, and for that purpose, it is effective to reduce a length of the lens. However, in general, the reduction in length of the lens leads to deterioration in the field curvature, the distortion, and the like.